A sealing structure is conventionally known which includes a resin backup ring provided adjacently to a seal ring formed of a rubber-like elastic body and at a position further toward a lower pressure side than the seal ring. In the sealing structure, the backup ring allows suppression of stick-out of a part of the seal ring from an annular groove in which the seal ring is installed. Consequently, possible damage to the seal ring can be suppressed. However, in general, a cut portion is formed in the backup ring at one position in a circumferential direction so as to facilitate installation. Thus, a part of the seal ring may be sandwiched between cut surfaces of the cut portion, leading to damage to the seal ring. Therefore, the seal ring still has room for improvement.